From the viewpoint of environmental protection, emission control of fuel vapor which can leak from fuel tanks to the outside has been recently strengthened as well as control of emission gas from engines mounted in vehicles. Especially, the standards laid down by the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) require that fuel vapor leaking from a minute opening in a fuel tank should be detected. For this purpose, a fuel vapor leakage check module is in wide use. The module is so designed that the interior of a fuel tank connected to a canister port through a canister is pressurized or depressurized and thereby any leakage of fuel vapor from the fuel tank is inspected. (U.S. Pat. No. 5,890,474 for example.)
Conventionally, various methods have been developed for the enhancement of the detection accuracy of fuel vapor leakage check modules. For example, a standard orifice corresponding to the aperture diameter for which fuel vapor leakage is allowed in a fuel tank is depressurized or pressurized to detect a standard pressure; thereafter, the interior of the fuel tank is depressurized or pressurized to detect its pressure, and the detected pressure is compared with the standard pressure to inspect fuel vapor leakage. FIG. 5 illustrates a fuel vapor leakage check module for implementing this method. The check module 1 in FIG. 5 is so constructed that: a standard orifice 5 is provided in a passage 4 connecting to a canister port 3 which can be opened to the air by a changeover valve 2; the passage 4 is depressurized or pressurized by a pump means 6.